In order to protect an occupant in an emergency or a collision of a vehicle, an airbag device (seat-in bag device) installed inside a seat is used. The airbag device includes an airbag disposed inside a seating portion of a seat. The airbag is inflated under the occupant and restrains the occupant who is wearing a seatbelt to the seat. The airbag device prevents a submarine phenomenon that usually occurs to the occupant when an impact is applied to the vehicle from the front. The submarine phenomenon is a phenomenon in which the lumbar of an occupant slips through a lap belt of a seatbelt and moves forward off the lap belt.
To counter the phenomenon, an occupant protecting device having an enhanced function to suppress forward movement of the lumbar is known (see PTL 1).
In an existing occupant protecting device, a front inflation portion of an airbag is inflated to a high position, under knees of an occupant. By doing so, the knees of the occupant are pushed above thighs, whereby the forward movement of the lumbar is suppressed.
However, in such an occupant protecting device, the airbag is formed by stacking base fabric pieces constituting upper and lower surfaces on top of each other and joining them. Therefore, the airbag in an undeployed state has a flat shape, and the airbag tends to be inflated uniformly over the entirety thereof. Furthermore, because the volume of the front inflation portion is limited, the height to which the knees are pushed up may be influenced. Depending on the state of the seat (such as the strength of a seat pad and the angle of a seating surface), it may be impossible to push up the knees high enough. In addition, because the shape and height of this airbag, in an inflated state, cannot be designed freely, the design flexibility is low. As a result, it is difficult to appropriately inflate the airbag so as to flexibly conform to a change of the state of the seat or the restraining conditions of the occupant. Thus, from the standpoint of improving the occupant protection function, further improvement of occupant protecting devices has been required so that forward movement of the lumbar can be more reliably suppressed.